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Dec. 12, 2013 -- It’s not unusual for people to say they feel much better after dropping gluten from their diets, even though they don't have celiac disease, digestive experts report.

What these people describe has come to be called “non-celiac gluten sensitivity,” or NCGS. It's a little-studied diagnosis that has contributed to the growing market for gluten-free products, expected to surpass $6.2 billion worldwide by 2018, according to one estimate.

NCGS is “a wildly popular topic on the Internet,” says Douglas Seidner, MD. “There’s a lot of discussion, a bit of confusion.”

Seidner is director of the Center for Human Nutrition at Vanderbilt University. He spoke about the gluten controversy last Saturday at a meeting of the American Society for Nutrition.

In celiac disease, eating a protein in wheat, rye, and barley called gluten triggers inflammation in the small intestines.

The problem is that little is known about NCGS, from how much gluten is needed to trigger symptoms to whether gluten is even the culprit, Seidner says. Meanwhile, he says, many people are eliminating gluten from their diets because they think it will improve their health, even if they don’t have celiac disease.

Celiac Disease vs. NCGS

About 1 in 100 people worldwide has celiac disease. The only treatment is a gluten-free diet. When someone with celiac disease eats even tiny amounts of gluten, their immune system attacks the the lining of the small intestine. This can lead to malnutrition.

No one knows how common NCGS is, Seidner says, but it could affect as many as 6 out of every 100 people. NCGS is a separate condition from celiac disease, and it’s not known if people with the former will ever go on to develop the latter, he says. Despite the condition’s name, “gluten may not be the only dietary compound in wheat” that leads to NCGS, Seidner says.

Both conditions have intestinal symptoms, such as bloating and pain, and symptoms outside the digestive tract, such as fatigue. A small percentage of people with irritable bowel syndrome have either celiac disease or NCGS as well, Seidner says.

Conflicting Findings

There are only three published studies to refute or support the diagnosis of NCGS, Seidner says.

Italian doctors who published a study last year of 920 of their patients concluded that some people who don’t have celiac disease are indeed sensitive to wheat. But some were sensitive to multiple foods as well as wheat, the researchers said.

The two other studies were done by a team of Australian scientists. They reached two different conclusions.

The first study, published in 2011, looked at people who didn’t have celiac disease but controlled their digestive tract symptoms with a gluten-free diet. Participants were randomly divided into two groups and were told to stick with their usual gluten-free diet. The researchers also gave everyone two slices of bread and one muffin to eat every day for up to 6 weeks. One group got gluten-free bread and muffins; the other got them with gluten.

Within 1 week, the group that ate the bread and muffins containing gluten reported more symptoms, such as pain and bloating, than the other group. “’Non-celiac gluten intolerance’ may exist,” the scientists concluded, but they found no clues as to why.

The Australians’ second trial, published this past May, found that symptoms in people with NCGS were just as severe on a gluten-free diet as on a high-gluten diet. The researchers provided the participants with all of their meals and also restricted dairy products, which can cause digestive tract symptoms. One possible explanation for their mixed results could be that they more tightly controlled what participants ate in their second study, Seidner says.

“We’re sort of left with some conflicting results and a dearth of information,” he says.

Getting Tested

People who think they have NCGS should get tested to make sure they don’t have celiac disease, according to the American College of Gastroenterology (ACG).

If they have the same fairly common genetic variations seen in celiac patients, they should take more tests to rule celiac disease out, starting with a blood test to look for elevated levels of certain antibodies, the ACG says. If the blood test and symptoms suggest celiac, the next step is a biopsy of their small intestine to confirm it, according to the college.

“If everything’s negative, they clearly don’t have celiac disease, but they may insist on staying on the diet,” Seidner says. He encourages such patients to talk to a dietitian.

Patients have to resume eating gluten for weeks before getting the blood test and biopsy, and some refuse, Charles Halsted, MD, said at the meeting. He specializes in intestinal and nutritional diseases at the University of California, Davis.

These findings were presented at a medical conference. They should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.